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Phase contrast microscope asbestos fiber counting.

Several comments were provided regarding aspects of NIOSH Method 7400, which is used in the monitoring for asbestos (1332214) particles. The current version was Revision 3, which introduced several changes to NIOSH 7400 in an effort to improve sensitivity and reproducibility. Sample integrity was an important part of providing accurate results. Any sample revealing indications of being mishandled or improperly taken should be rejected. Any filter appearing torn or soaked with water should be rejected. It was noted that the optimum fiber loading range for NIOSH Method 7400 is 100 to 1300 fibers per square millimeter of filter surface area. Under no circumstances should a number be reported that is below the limit of detection. NIOSH Method 7400 estimated a limit of detection of 5.5 fibers counted or seven fibers per square millimeter of fiber surface area. The Method indicated that if more than 50% of the filter was covered with particulates, the sample was too overloaded to count and should be rejected. This Method has more quality assurance procedures than any other NIOSH analytical method to ensure that counts are repeatable and reproducible within the United States.